Merge: Remove unportable assumption about struct layout (Bug#8884).

* buffer.c (clone_per_buffer_values): Don't assume that
sizeof (struct buffer) is a multiple of sizeof (Lisp_Object).
This isn't true in general, and it's particularly not true
if Emacs is configured with --with-wide-int.
* buffer.h (FIRST_FIELD_PER_BUFFER, LAST_FIELD_PER_BUFFER):
New macros, used in the buffer.c change.